The US-led coalition fighting IS has told its local Syrian allies that they must be exclusively focused on fighting ISIS and not fight the Syrian regime, a directive that prompted one rebel group to depart a joint coalition base in Southern Syria, several coalition and US defense officials told CNN.

"The coalition supports only those forces committed to fighting ISIS," coalition spokesman US Army Col Ryan Dillon told CNN.

But one US-backed group which calls itself "the Shohada Al Quartyan" has balked at the restriction, opting to leave the base to carry out independent operations against Syrian regime troops several US and coalition officials told CNN. The officials added that other local allies remain at the garrison and continue to cooperate with coalition advisers.

The tensions between the rebel group and the US military over fighting Assad comes days after the head of Special Operations Command, Gen Tony Thomas, appeared to confirm a Washington Post report that the Trump Administration had ended a classified CIA program to arm anti-Assad rebels in Syria.

In another development, a ceasefire deal has been reached that will see jihadist fighters withdraw from the Syria-Lebanon border, a top Lebanese official said yesterday after a week-long operation there by the Hezbollah movement.

The powerful Shia militant group began the assault on fighters from al-Qaeda's former Syrian affiliate, previously known as the Al-Nusra Front, in the Jurud Arsal border region last week.

The report came as Syrian Observatory for Human Rights said a US-backed alliance has ousted Islamic State group jihadists from half of their Syrian bastion Raqa.

The Syrian Democratic Forces have been fighting for several months to capture the northern city, which has become infamous as the Syrian heart of IS's so-called "caliphate."

The SDF "are now in control of 50 percent of Raqa city despite the fierce resistance mounted by IS", Rami Abdel Rahman, head of the observatory, told AFP.

The SDF assault has been backed by air strikes, special forces advisers, equipment and weapons from the US-led coalition fighting IS in Syria and neighbouring Iraq.

On Wednesday, a barrage of US-led air strikes across the city left at least 29 civilians dead, the British-based Observatory said.

Meanwhile, President Vladimir Putin has signed a law ratifying a deal with the Syrian government allowing Russia to keep its air base in Syria for almost half a century, official documents show.

The original deal, signed in Damascus in January, sets out the terms under which Russia can use its Hmeymim air base in Latakia Province which it has used to carry out air strikes against forces opposing President Bashar al-Assad.

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